1. Field of the Invention
The present invention relates to a vibration sensing element and a vibration sensor using the vibration sensing element which are suitable for sensing mechanical vibration occurring in an object made of glass, metal or the like, particularly the characteristic mechanical vibration occurring when such an object breaks.
2. Prior Art
Vibration sensors for converting the characteristic mechanical vibration occurring upon the breakage of a glass plate, metal plate or the like into an electrical signal are widely used in, for example, security systems. As taught by Japanese Utility Model Publication No. Sho 62-39501, the vibration sensing element used in the prior-art vibration sensor of this type is a normally closed switch comprising a pair of conductive metal plate springs facing each other across a small distance with contacts at their tips touching lightly. This normally closed switch responds to vibration produced by breakage of the glass plate or the like (secured object) by repeatedly turning on and off (making and breaking) at high frequency. The normally closed switch is series-connected within a prescribed closed loop circuit for converting the vibration occurring when the secured object breaks into an electrical pulse signal with short-period level variation.
The conventional vibration sensor is equipped with such a vibration sensing element constituted as a normally closed switch having a pair of conductive metal plate springs touching at light contact pressure and is configured to utilize the electric signal produced by the vibration sensing element when it periodically turns on and off in response to vibration. Owing to this configuration, the sensor sensitivity to vibration can be increased only by lowering the contact pressure of the normally closed switch. When the contact pressure of the switch is reduced, however, the increase in the on-state resistance of the contacts owing to dirtying, oxidation, condensation, moisture and the like tends to be more pronounced. This degrades the operational reliability of the switch and makes it incapable of operating dependably over long periods.
This problem can be mitigated by hermetically sealing at least the contact portion of the normally closed switch. This has not been a practical solution, however, because with the conventional hermetically sealed contact configuration, measurement of the contact pressure of the normally closed switch is possible only before sealing of the switch contacts. After sealing of the contacts, measurement to determine whether the contact pressure is at the rated value cannot be made accurately from the exterior. This is extremely inconvenient from the point of quality control.
Moreover, the vibration sensor utilizing the vibration sensing element of this type is required to have high reliability and maintain its rated performance over a long period. The switch section therefore has to be tamperproof. The conventional practice has therefore been not to seal the switch contact portion but to adopt a sealed structure for the vibration sensor casing that houses the vibration sensing element and other components. Various techniques are used to make the casing difficult to force open. As a result, the prior-art vibration sensor has a complex structure and is expensive to manufacture.